Moraxella catarrhalis (previously called Neisseria catarrhalis or Micrococcus catarrhalis) is a Gram-negative aerobic bacterium that is a member of physiological bacterial flora of the mucous membranes of the upper respiratory tract. For many years, Moraxella catarrhalis was considered to be a non-pathogenic species that is the human commensal of the upper respiratory tract. Recent studies have shown that this bacterium is a pathogen and, in favorable conditions, can lead to infection. It is estimated that 75% of children and 1 to 5% of healthy adults are carriers of M. catarrhalis.
Moraxella catarrhalis is the third most frequently occurring bacterial agent responsible for acute otitis media and sinusitis in children (except for Streptococcus pneumoniae and Haemophilus influenzae). Infections also exacerbate chronic bronchitis and pneumonia. In adult patients, Moraxella catarrhalis is most often responsible for laryngitis, acute bronchitis and pneumonia. These infections are particularly dangerous for patients suffering from chronic obstructive pulmonary disease, as they lead to an exacerbation of the disease.